Bellari, a U.S. company based in Utah has been making inexpensive tube based mic preamps for a number of years. The cosmetics have changed slightly on the various units through the years but the basic design and function remains the same. The latest version, the RP520 sports newer style knobs and VU meters, as opposed to the previous version which had rather cheap looking and feeling knobs, and LEDs instead of VUs. Other than that, it's the same unit as the previous version.
Lets talk about the positives first. Bellari preamps are a full fledged, "true" tube design. By "true" tube design, I mean the input is transformer coupled and the tube is running at a full 240 volts. These preamps were made in the good ol USA and one of the things that users really like and that has been discussed quite a bit on the web is the ability to modify them easily. In fact, the Bellari preamps were made to be easily modded. The stock Chinese tubes can be swapped for something better of course, but that's not all. The op amps on the Bellari units are socketed, meaning they are also easily exchanged for higher quality ones. Cosmetically, the RP520 looks nice and it's very easy and intuitive to operate.
Now the negatives: the build quality on the Bellari units is in general pretty poor. I've owned quite a bit of made in China gear that was built much better. One example: I once bought a single channel version of this preamp that's now discontinued. It came with the top on backwards, so that the slots for air ventillation of the tube were not where they were supposed to be! Another unit I demoed was microphonic. Not just the tube, the WHOLE UNIT! When gain and output was turned up to a normal level, tapping on or just brushing the front with my hand would be amplified like the whole thing was a guitar pick up! The input transformers on the Bellari units are pretty cheap and easily overload with even a moderately hot input signal. They don't sound pretty when driven to this point. As far as the overall sound, it's very vintage and one can get pretty good even order harmonics going. Clarity and definition however, are not it's forte and I suspect this has something to do with the fact that the input impedance of the RP520 is a mere 600 ohms. You can't use this preamp with a ribbon mic because of the low input impedance. Overall, the RP520 looks good on paper, but is pretty limited in it's usefulness.

I used to have a Bellari RP520 Stereo Tube Preamp until last week. It was not easy, but I had to sell it, to give space to some new stuff in my rack. I have used it for about ten years and it never let me down. That’s probably why I developed feelings related to it.

The RP520 is the most beautiful dirty-sound pre I ever tasted. To be honest, it was the only “really dirty” tube sounding pre I know. It seems to add some crispy harmonics in the mid / low-mid frequencies regions. I mean, I never did a technical test, but that’s what I fell when listening to any instrument passing through it. If I need to warm up sound giving some Lo-Fi approach, the Bellari would be my first choice.

As most tube preamps, it has an input and output gain knobs and a phase button for each channel. It also has two convenient -30dB pad buttons, one in the front panel (for the input) other in the back (for the output). There are TRS line inputs in the front, XLR microphone inputs and the option to use one XLR or two TRS (never understood why) outputs in the back. The front panel also brings one VU meter per channel. Mine never stopped working, but the VU lights seem to be alive: sometimes they function, sometimes not.

I don’t like the fact Bellari putted phase and output pad in the back panel. This makes me hating them when I remember I sometimes had to reach the back panel or give up to use those controls.

Another curious stuff is that the RP520 seems to have a different gain structure then most preamps in the market. I have shown this for some friends who also had to spend sometime with it to get its way to work. But not a big deal, once you got it, it’s a piece of cake.

Resumé: not Hi-fi, almost Lo-Fi, dirty sounding, easy to modify, unique but cheap.

I have the Bellari RP503. I use it for guitar and bass cabs and have even used it on vocals just to try her out. She ain't the purdiest girl at the ball but I think I paid like $75 for this 1u strip and she performs very nicely! Transparent and clean, I love the compressor and filament lit analog Gain Reduction VU meter for compression. Very cool. Nice 30db+ on the pre and 3 band eq for even more control. I haven't changed out the tube or anything else but I'm sure it can be made to sound much better. I just picked up an apollo 8duo and focusrite octopre MKII dynamic so i won't be using this much but it's always nice to have extra pre's around that you can count on for decent sound if you are tracking a very large session, this is a handly extra to have in the closet if need be! If I were looking for a very cheap alternative to ART or MXL or something in the Behringer price range, I would certainly go Bellari for the true tube components and easy compression. Very cool little pre for sure